In general, acrylonitrile-butadiene-styrene (ABS) copolymer resins have superior processability and excellent appearance characteristics due to inclusion of styrene, rigidity and chemical resistance due to inclusion of acrylonitrile, and impact resistance of butadiene rubber. Accordingly, acrylonitrile-butadiene-styrene (ABS) copolymer resins are used in various fields such as automobile interior materials, housings for household appliances, and toys. In particular, since automobile interior materials are often exposed to high temperatures, superior thermal resistance, e.g., heat deflection temperature (HDT), is required.
To produce acrylonitrile-butadiene-styrene copolymer resin satisfying such high thermal resistance, a method of melt-mixing an α-methyl styrene-acrylonitrile copolymer (AMSAN), which is prepared through emulsion polymerization or solution polymerization and has a high glass transition temperature (Tg), with a rubber-reinforced graft copolymer (ABS copolymer), which is prepared through emulsion polymerization, while controlling the content of rubber within a predetermined range has been suggested.
Such an α-methyl styrene-acrylonitrile copolymer is generally prepared by copolymerizing an α-methyl styrene monomer with a vinyl cyanide compound monomer through emulsion polymerization. However, the α-methyl styrene monomer has drawbacks such as low reactivity and polymerization stability. When the content of α-methyl styrene is increased to overcome such disadvantages, a polymerization conversion rate is decreased, and thus, productively is decreased. In addition, problems such as depolymerization and oligomer generation may occur depending upon a temperature condition during polymerization, whereby heat resistance may be decreased.